For a crosshead-type diesel engine, a cylinder oil for lubricating the friction point between the cylinders and pistons and a system oil for lubricating and cooling other portions have been used. The cylinder oil is required to have a suitable viscosity needed to lubricate a friction portion between a cylinder and a piston (piston ring) and a function to maintain detergency needed to allow a piston and a piston ring to move appropriately. Furthermore, since for this engine, a high-sulfur content fuel is generally used for the economy reason, it has a problem that acidic components such as sulfuric acid generated by the combustion of the fuel corrode a cylinder. In order to avoid this problem, the cylinder oil is required to have a function to neutralize the acidic components such as sulfuric acid so as to prevent corrosion.
Meanwhile, for the purpose of further improving properties, a recent crosshead-type diesel engine tends to be directed toward increases in the cylinder diameter (for example, 70 cm or greater bore size), in the piston stroke (for example, ultra long stroke such that it is 8 m/s or more at an average speed) and in the combustion pressure (for example, 1.8 MPa or greater brake mean effective pressure (BMEP)), resulting in an increase in the temperatures of the piston and cylinder wall. The increase in the combustion pressure involves the dew point rise of sulfuric acid causing a cylinder to be likely to corrode by sulfuric acid. Furthermore, for the measure of inhibiting the corrosion by sulfuric acid, the cylinder wall temperature tends to be increased (for example, 250° C. or higher cylinder wall temperature) and also the amount of a lubricating oil to be lubricated into a cylinder has been decreased. The circumstances concerning the lubrication of a cylinder has become remarkably severer. As such change of the circumstances, the anti-scuffing properties of a lubricating oil has been required to be urgently improved (Patent Literature 1, Patent Literature 2).
The cylinder oil is a lubricating oil that is of a once-through type and thus has never been considered in respect of oxidation stability (Patent Literature 1, Patent Literature 2). The inventors of the present application have found that addition of specific antioxidant can improve significantly not only the antioxidation properties but also the anti-scuffing properties of a cylinder oil. Meanwhile, the oxidation stability of a lubricating oil is known to be improved by using a base oil with a less aromatic component or adding an antioxidant. Molybdenum compounds are also known to be act as antioxidant (Patent Literature 3, Patent Literature 4). Patent Literature 3 discloses that a crank case oil comprising a hydrocracked base oil, an oil-soluble molybdenum compound from which sulfur is removed, an oil-soluble diarylamine and a phenate of an alkaline earth metal is excellent in oxidation stability and decreases the wear of a tappet and the deposits on rings and valves. Patent Literature 4 discloses that a lubricating oil comprising a base oil with an aromatic content of 3.0 percent by mass or less, alkyldiphenylamines and/or phenyl-α-naphthylamines and sulfurized oxymolybdenum dithiocarbamate and/or sulfurized oxymolybdenum organophosphorodiate has high heat-resistnce and oxidation stability and low friction properties.